A career man or woman, a member or the rat race, someone whose life and personality are defined by work. Their temperament is the same at all times, and they are unable to turn their serious nature off and relax, hence the term 'stiff'.

Alex is sacrificing his life working ridiculous hours just to earn his obscene bonus every year, so that he can be wealthy in old age. But by age 40 he had become a corporate stiff, forever changed by years of drudgery that have made him unemotional and boring.